Washington (AP) - Delaware Republicans call Senate hopeful Christine O'Donnell a liar who "could not be elected dog catcher" in a fierce attack that underscores GOP fears of the tea party-backed candidate knocking off top recruit Rep. Mike Castle and winning the nomination.

Stunned by tea partier Joe Miller's upset of Alaska Sen. Lisa Murkowski, Republicans are taking no chances in Delaware, which holds its primary Sept. 14. The party sees Castle, the state's lone congressman since 1993, as the best candidate for the seat long held by Vice President Joe Biden.

Republicans circulated audio of a testy, 22-minute interview that O'Donnell had with radio station WGMD on Thursday. Party officials also have said she inflated her resume and made flat-out untrue statements while being dogged by questions about tax liens and foreclosures. Castle says she has misrepresented his record.

"She's not a viable candidate for any office in the state of Delaware," state Republicann Party Chairman Tom Ross, who is backing Castle, said in a telephone interview. "She could not be elected dog catcher."

Castle's campaign began running radio ads on Thursday and prepared negative television ads against O'Donnell. The radio ads cite a News Journal of Wilmington, Del., report on O'Donnell's finances and declares her "a financial nightmare."

"According to the News Journal, O'Donnell has become a professional candidate, living off contributions from her two failed campaigns," the announcer says. "While leaving a trail of unpaid bills to vendors and staff, O'Donnell instead used those donations for her own personal expenses and rent. ... While you work hard to pay your bills and taxes, Christine O'Donnell lives by different rules."

The ad also cites O'Donnell's slow repayment of student loans and that she "got caught when she refused to pay thousands in taxes she owed the IRS and defaulted on her mortgage."

The planned television commercials would air in the week leading up to the primary. The campaign also has created a website, RealChristine.com, a clearinghouse of negative O'Donnell stories.

"Unfortunately, the truth always seems to be an issue," said Ross. "Her version of reality doesn't jibe with any of the facts."

O'Donnell's campaign did not return messages seeking comment.

The Tea Party Express has announced a six-figure commitment to back O'Donnell. The group announced its first expenditures -- more than $46,000 -- late Thursday and put the anticipated cost at about $250,000.

"We are launching an aggressive multimedia and multiplatform campaign to help propel Christine O'Donnell to victory, and we've only just begun," said Amy Kremer, chairman of the Tea Party Express.

In the radio interview Thursday, O'Donnell refused to back down from claims that she won two of Delaware's three counties in her 2008 Senate bid against Biden, despite numbers that show she didn't.

"I was the 2008 endorsed candidate against Joe Biden and I won in two counties," O'Donnell had told a group in Pennsylvania.

WGMD's Dan Gaffney, a conservative radio host who backed O'Donnell's Senate bid, asked her to explain the claim.

"Look at the results," O'Donnell said. "What do they say? 49 (percent), 49. I call that a tie."

She lost that county by 272 votes.

Commenting on Kent County, home to state capital Dover, O'Donnell said: "I said I nearly tied."

The host played the audio again that shows she didn't couch it that way. Flustered by the questioning, O'Donnell asked Gaffney whether he was being paid off by Castle, who has refused to debate her.

She then blamed her schedule for the missteps. "You're on the campaign trail, starting at 5 a.m., you go to 12 ... you go until midnight," she said. "Sometimes you slip up on those things."

Although private polling shows Castle with a comfortable lead, they want to avoid a surprise like Miller upending Murkowski.

Ross argued that the two candidates -- O'Donnell and Miller -- are far different.

"When you look at Joe Miller, he's an Ivy League graduate, a war hero and an attorney who is prominent in the community," he said. "We could go across the street from the apartment Christine O'Donnell rents and we probably couldn't find anyone who knows her."

Although O'Donnell has appeared on the ballot in the past, she faces an uphill race against Castle, Delaware's sole representative in the U.S. House who won in 2008 with 61 percent of the vote.

Republicans also take some comfort in the calendar. The deadline to register in the Republican primary has passed; tea party activists energized by the upset in Alaska missed their chance to vote in the closed primary. The Republican winner will face Democrat Chris Coons.

Delaware Republicans call Senate hopeful Christine O'Donnell, the tea party candidate, a liar who 'could not be elected dog catcher.'